Inclusion Criteria:

- Histologic diagnosis of adenocarcinoma of the prostate

- Metastatic prostate cancer (positive bone scan or measurable disease)

- Total testosterone of greater than 50 ng/dL, except for patients with prior
orchiectomy, where testosterone does not need to be measured.

- Patients who are receiving an antiandrogen as part of primary androgen ablation must
demonstrate disease progression following discontinuation of antiandrogen and
completion of a washout period and then observe disease progression.

- Patients must stop using any herbal product known to decrease PSA levels (eg., saw
palmetto and PC-SPES) or any systemic or topical corticosteroid at least 4 weeks
prior to screening. Progressive disease must be documented after discontinuation of
these products.

- Progressive disease after androgen deprivation (or hormone therapy). For patients
with measurable disease, progression will be defined by Response Evaluation Criteria
in Solid Tumors (RECIST) criteria. For patients with progression in, or without any
measurable disease, a positive bone scan and elevated PSA will be required.

- Patients receiving bisphosphate therapy must have been on stable doses for at least 4
weeks with stable symptoms prior to enrollment.

- No prior chemotherapy or immunotherapy (tumor vaccine, cytokine, or growth factor
given to control prostate cancer).

- Prior radiation therapy completed at least 4 weeks prior to enrollment. No prior
radiopharmaceuticals (strontium, samarium) within 8 weeks prior to enrollment.

Exclusion Criteria:

- Bone pain due to metastatic bone disease severe enough to require routine narcotic
analgesic use.

- History of severe hypersensitivity reactions to drugs formulated with polysorbate 80.

- Patients with active autoimmune disease or a history of autoimmune disease that
required systemic steroids or immunosuppressive medications, except for patients with
vitiligo.

- Prior therapy with any anti-cytotoxic T-lymphocyte antigen 4 (anti-CTLA-4) antibody.

- Active infection requiring therapy.

- Concurrent medical condition requiring the use of systemic or topical
corticosteroids; systemic or topical corticosteroids must be discontinued at least 4
weeks prior to enrollment. The use of inhaled corticosteroids is acceptable.